The invention relates to a two to four wire hybrid circuit arrangement for a pulse code modulated time multiplex telecommunications system. The hybrid circuit arrangement couples a two-wire extension line to a four-wire transmission line and, more specifically, has a receiving and a transmitting branch, each branch including a filter with low-pass characteristics, a receiver filter and a transmitter filter.
Hybrid circuit arrangements for telecommunication systems are very well known in the art. Such a circuit arrangement couples a subscriber set or more generally a two-wire extension line to a four-wire transmission line. A main element is a transformer which has to transform output voice signals from the two-wire extension line to a transmitting branch of the four-wire transmission line or to couple input voice signals from the receiving branch of the four-wire transmission line to the two-wire extension line. The principal problem of such a hybrid circuit arrangement is to perform good transmission characteristics in both signalling directions without undue loss while having a high attenuation from the receiving branch to the transmitting branch of the four-wire transmission line.
Another problem in designing such hybrid circuit arrangements is that a non-real line impedance is present on the two-wire side. Well known hybrid circuit arrangements have a frequency-dependent attenuator to compensate this influence. In known hybrid circuit arrangements there is provided on the four-wire side a balancing network comprising among other things, a relatively high additional inductance. Known hybrid circuit arrangements, therefore, have several disadvantages. As pointed out, they have different inductive or capacitive reactances as part of a frequency-dependent attenuator, of a balancing network or of a power supply bridge. Those reactances are relatively difficult to realize as integrated circuits. Circuit arrangements composed of discrete elements, however, are undesirable for reasons of cost and space.